The invention relates to a frequency-shift-keyed (FSK) data receiver comprising a mixer stage in which a received FSK signal, having a carrier frequency f.sub.c which is frequency-modulated by a data signal to produce a given frequency swing. .DELTA.f, and a local oscillator signal produced by a voltage-controlled oscillator with a signal frequency f.sub.L located within the band of the receiver, are mixed. Relative to the carrier signal frequency f.sub.c, the local oscillator frequency is shifted through a given value .delta.f. A bandpass filter is connected to the mixer stage, a detection circuit is connected to the bandpass filter for recovering the data signal from the sum and difference-frequency signals .DELTA.f.+-..delta.f produced by the mixer stage, and an AFC control loop is connected between the detection circuit and the voltage-controlled oscillator.
Such a receiver is disclosed in United Kingdom patent application No. 8132181, to which U.S. Pat. No. 4,523,324 corresponds. In this receiver the sum and difference signal frequencies are filtered in separate filters after having passed the bandpass filter and are applied to a differential amplifier. This differential amplifier checks whether at that moment a high or a low signal level of the transmitted data signal is received.
The AFC control loop comprises a mixer stage to which a separate oscillator having a signal frequency equal to the frequency swing .DELTA.f is connected. The sum and difference signal frequencies applied to this mixer stage are down-transformed, whereafter the signal component having the given frequency value .delta.f is obtained with the aid of a low-pass filter. This signal component is converted into a control voltage for the voltage-controlled oscillator with the aid of a frequency-voltage converter.
This receiver has the disadvantage that at least three sharp filters are used which, for production in accordance with integrated circuit techniques requires many external capacitors and a correspondingly large number of connections. In addition, the AFC loop comprises an additional oscillator. In the receiver a controllable intermediate-frequency amplifier having a large dynamic range is required, which necessitates a large amount of current, which makes integration still more difficult. AM noise is not suppressed. Finally, the AFC loop used does not work satisfactorily with small input signals which however still have an adequately large S/N ratio to enable adequate detection. The reason for this AFC failure is that for such small input signals the loop can lock onto several frequencies, more specifically onto noise signals.
In certain applications such as pagers very severe requirements are imposed on the sensitivity, the selectivity and the consumed power. Thus, in England a sensitivity of 10 .mu.V/m, a selectivity of -65 dB at .+-.25 kHz and a power dissipation less than 6 mW is required for an aerial having a length of 3 cm in the frequency band for pagers from 148-152 MHz.